Such a casing comprises a shroud of generally frustoconical shape which carries fastener hooks on its inside face, i.e. facing towards the inside of the turbine, for holding one or more turbine nozzles formed by rings or ring segments carrying stationary vanes. Rotary wheels carrying the moving blades of the turbine rotor are located between such nozzles. Each pair constituted by a nozzle and a rotary wheel constitutes a stage of the turbine.
Turbines, and more particularly airplane turbojet turbines, have very hot combustion gases passing through them at temperatures that can sometimes be as great as 1100° C. in certain turbine stages. The shroud of the casing is thus subjected to very significant heating and it is necessary to cool it using a cooling system.
The European patent application published under the No. 1 288 444 A1 gives an example of a cooling system constituted by perforated annular ducts that are fed with cool air and that surround the shroud. The cool air is blown against the outside face of the shroud, through the perforations in the ducts. Nevertheless, in practice, it has been found that such a cooling system requires equipment that is bulky and expensive and does not enable the heating of the shroud to be reduced significantly at its inside face.
In order to reduce the bulk of ducted cooling systems and improve the cooling of the inside face of the shroud, double-walled or double-skinned shrouds have been devised that present empty spaces between their inner and outer skins, and cool air is caused to flow through those spaces. In order to make such shrouds, use is made of conventional casting techniques using cores that are soluble. More precisely, a molten alloy is cast into a mold whose walls have the shape of the outlines of the shroud, with soluble cores being positioned inside the mold. Once the alloy has cooled and solidified, the shroud is extracted from the mold and is dipped in a basic dissolving bath so that the cores dissolve, thereby leaving free said empty spaces.
That method of manufacture nevertheless presents certain drawbacks associated with the very nature of manufacture by casting. Thus, it is difficult to control the dimensional precision of the shroud casting. In addition, mechanical properties such as elastic limit and fatigue resistance of the resulting shroud are not good, often because of its high degree of porosity or because of the presence of structural defects in its bulk that weaken it (these defects are often associated with the casting cooling in non-uniform manner).